1. Field of the Invention
The disclosure relates to a liquid crystal display device, and more particularly to a backlight unit for a liquid crystal display device including light-emitting diodes and a driving method of the same.
2. Discussion of the Related Art
According to the rapid development in information technology, flat panel display (FPD) devices having thin thickness, light weight, and lower power consumption, such as liquid crystal display (LCD) devices, plasma display panels (PDPs), electroluminescent display (ELD) devices and field emission display (FED) devices, have been developed and have replaced cathode ray tubes (CRTs).
Among these devices, liquid crystal display (LCD) devices are most widely used for monitors of notebook computers, monitors of personal computers and televisions due to excellent moving images and high contrast ratio. By the way, the LCD devices require an additional light source because the LCD devices are not self-luminescent. Therefore, a backlight unit is disposed at a rear side of a liquid crystal (LC) panel and emits light into the LC panel, whereby discernible images can be displayed.
Backlight units are classified into an edge type and a direct type according to the position of a light source with respect to a display panel. In edge-type backlight units, one or a pair of lamps are disposed at one side or each of two sides of a light guide panel of a backlight unit. In direct-type backlight units, a plurality of lamps are disposed directly under a display panel.
FIG. 1 is a cross-sectional view of a liquid crystal display (LCD) device including a direct-type backlight unit according to the related art. In FIG. 1, a related art LCD device includes a liquid crystal panel 10, a backlight unit 20, a support main 30, a top cover 40 and a cover bottom 50. The liquid crystal panel 10 includes first and second substrates 12 and 14. The backlight unit 20 is disposed over a rear surface of the liquid crystal panel 10. The support main 30 of a rectangular frame shape surrounds side surfaces of the liquid crystal panel 10 and the backlight unit 20. The top cover 40 covers edges of a front surface of the liquid crystal panel 10, and the cover bottom 50 covers a rear surface of the backlight unit 20. The top cover 40 and the cover bottom 50 are combined with the support main 30 to thereby constitute one-united body.
The backlight unit 20 includes a reflection plate 22, a plurality of lamps 24, a diffuser plate 26, and a plurality of optical sheets 28. The plurality of lamps 24 are disposed over the reflection plate 22, and the diffuser plate 26 and the plurality of optical sheets 28 are sequentially disposed over the plurality of lamps 24.
A couple of side supports (not shown) are combined with the cover bottom 50 to fix and support the plurality of lamps 24. The lamps 24 may be cold cathode fluorescent lamps (CCFLs) or external electrode fluorescent lamps (EEFLs).
Recently, to display more vivid images, a method of driving the backlight unit 20 of the LCD device, in which the lamps 24 sequentially turn on/off, has been suggested, and the method may be referred to as a scanning driving method. According to this, the contrast ratio is improved by increasing the brightness of bright images or decreasing the brightness of dark images, and more vivid images are displayed.
In addition, the LCD devices have been generally used for desktop computer monitors and wall-mounted televisions as well as portable computer monitors, and LCD devices displaying vivid images and having a thin thickness have been widely researched.
Therefore, a method of reducing a distance A between the lamps 24 and the diffuser plate 26 of the backlight unit 20 has been suggested and developed to provide a thin LCD device.
However, to provide the liquid crystal panel with a high quality surface light source, various optical designs are considered. Among the designs, it is important to appropriately maintain the distance A between the lamps 24 and the diffuser plate 26. That is, when the distance A between the lamps 24 and the diffuser plate 26 is considerably short, light emanated from the lamps 24 has strong straightness, and the shapes of the lamps 24 are shown in a displayed image. Accordingly, lamp mura defects of a striped shape are caused. Thus, the image quality of the liquid crystal display device is lowered.